Sunglasses, which suppress glare, may be made with films having moderate neutral polarizing efficiency. Sunglasses, which selectively filter transmitted light, may be made with multilayer thin-films, diffractive structures that selectively reflect light or dyes with chromatic absorption. Most of these sunglasses have spectral profiles with a small first derivative such that the slope at any point in the transmission spectrum is shallow. Such spectra are generally suitable to produce a desired hue and photopic transmission. Other sunglasses have spectral profiles with more aggressive slopes. For example, certain blue blocking sunglasses made from suitable dyes strongly suppress blue transmission, improving sharpness at the expense of color balance. Other sunglasses block one band of inter-primary light (500 nm cyan or 580 nm yellow light) and may partially block the other band of inter-primary light with rare-earth doped glass or with multi-layer coating technology. Unfortunately, rare-earth doped sunglasses have a fixed spectrum and multi-layer coating sunglasses may have an objectionable image due to light reflection. Multi-layer sunglasses may also suffer from angle sensitivity effects.
Glasses including dielectric mirrors to enhance the vision of those with color vision deficiency are known. As with other multiple layer coatings, glasses with dielectric mirrors may have an objectionable image due to light reflection and may suffer from angle sensitivity effects.
Retarder stacks as described in the present application are N linear retarders that have been designed using Finite Impulse Response (FIR) filter techniques, wherein the impulse response of the N retarders generate at least N+1 output impulses from a polarized impulse input. As such, a placing retarder stacks between neutral polarizers forms FIR filters, and these FIR filters can be designed using standard signal processing methods. The amplitude of each responsive, output impulse is determined by the orientations of the retarders (and the analyzing polarizer) relative to the input polarization of the input impulse. Algorithms are used to determine the orientations of the retarder films and the analyzing polarizers so as to satisfy a prescribed impulse response. Further details of the design approaches for the described retarder stacks are described in greater detail in U.S. patent application Ser. No. 09/754,091, which is hereby incorporated by reference herein.